During normal operation of a vehicle, the seatbelt rests loosely on a vehicle occupant's body for reasons of comfort and the layers of belt webbing on the winding shaft of the automatic belt retractor are wound relatively loosely. In the event of an accident there is a risk that the vehicle occupant will be displaced forwardly in spite of the blocking of the winding shaft of the seatbelt retractor. It is known that to avoid the risk of excessive forward displacement of the vehicle occupant, prior to blocking of the winding shaft the belt slackness may be eliminated by a belt tightener. The known belt tighteners act, for example, on the belt buckle or on the winding shaft of the automatic belt retractor.